In stunning guns of which we are aware, the guns utilize a moveable bolt which is placed at the temple of the animal and propelled into stunning contact by the firing of a blank (no projectile) cartridge. The cartridge is loaded into a breech mechanism which is positioned to direct the explosive force of the fired cartridge against one end of the bolt. The spent cartridge case can be extracted from the breech for reloading. A typical heretofore successful gun design is that described in U.S. Pat. No. 4,625,442 issued Dec. 2, 1986 ("the '442 patent"), the disclosure of which is incorporated herein by reference, and marketed as the Koch Magnum .25 Stunner by Koch Suppliers, Inc. of Kansas City, Mo.
In recent years, there has been an effort to get away from the use of cartridges which have lead or barium containing propellants, which present adverse air pollution and other problems. Cartridges utilizing a plastic explosive rather than the conventional powders have been developed. One characteristic of the plastic is, however, that its firing results in a myriad of fine glass-like particles being discharged from the open end of the cartridge case.
It has been discovered that the newer plastic loads have consequences for the stunning gun that shorten the effective life of the breech mechanism, in particular. Apparently the presence of the glass-like particles emanating from the open end of the cartridge creates erosion of the breech surface adjacent to the open end and creates as a result of repeated firings an annular depression adjacent the open end. As the depression increases in width and depth it provides a space into which the casing portion adjacent to the open end (or least some part of it) can expand outwardly and into the eroded area. This expansion and deformation of the casing into the irregularity creates a locking effect which interferes with extraction of the spent casing from the breech to the extent that the guns affected become effectively inoperable.